The invention relates to supertwist liquid-crystal displays (SLCD) having very short response times and good steepnesses and angle dependencies, and to the novel nematic liquid-crystal mixtures used therein.
SLCDs as defined in the preamble are known, for example from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 098 070 A2; M. Schadt and F. Leenhouts, 17th Freiburg Congress on Liquid Crystals (8-10.04.87); K. Kawasaki et al., SID 87 Digest. 391 (20.6); M. Schadt and F. Leenhouts, SID 87 Digest 372 (20.1); K. Katoh et al., Japanese Journal of Applied Physics, Vol. 26, No. 11, L 1784-L 1786 (1987); F. Leenhouts et al., Appl. Phys. Lett. 50 (21), 1468 (1987); H. A. van Sprang and H. G. Koopman, J. Appl. Phys. 62 (5), 1734 (1987); T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984), M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (5), 236 (1987) and E. P. Raynes, Mol. Cryst. Liq. Cryst. Letters Vol. 4 (1), pp. 1-8 (1986). The term SLCD here covers any more highly twisted display element with a value for the twist angle of between 160.degree. and 360.degree., such as, for example, the display elements of Waters et al. (C. M. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3rd Intern. Display Conference, Kobe, Japan), STN-LCDs (DE-A 35 03 259), SBE-LCDs (T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021), OMI-LCDs (M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987), 236, DST-LCDs (EP-A 0 246 842) or BW-STN-LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)).
SLCDs of this type are distinguished, in comparison to standard TN displays, by significantly better steepnesses of the electrooptical characteristic line and consequently better contrast values, and by significantly less angle dependence of the contrast. Of particular interest are SLCDs having very short response times, in particular also at relatively low temperatures. In order to achieve short response times, the rotational viscosities, in particular, of the liquid-crystal mixtures were hitherto optimized using usually monotropic additives having relatively high vapor pressure. However, the response times achieved were not adequate for all applications.
In order to achieve a steep electrooptical characteristic line, the liquid-crystal mixtures should have relatively large values for K.sub.33 /K.sub.11 and relatively small values for .DELTA..epsilon./.epsilon..perp.. The ratio K.sub.33 /K.sub.11 (bend elastic constant/splay elastic constant)mainly determines the steepness of the electrooptical characteristic.
In addition to optimization of the contrast and the response times, further important requirements are made of mixtures of this type:
1. A broad d/p window PA1 2. High long-term chemical stability PA1 3. High electrical resistance PA1 4. Low frequency dependence and temperature dependence of the threshold voltage. PA1 d: Layer thickness (separation of the plane parallel outer plates); PA1 p: Natural pitch of the chiral nematic liquid-crystal mixture. PA1 two plane-parallel outer plates which, together with a frame, form a cell, PA1 a nematic liquid-crystal mixture of positive dielectric anisotropy which is present in the cell, PA1 electrode layers with superposed alignment layers on the insides of the outer plates, PA1 a pitch angle between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from about 1 degree to 30 degrees, and PA1 a twist angle of the liquid-crystal mixture cell from alignment layer to alignment value of between 100 and 600.degree., a nematic liquid-crystal mixture comprising PA1 characterized in that component T comprises at least one compound of the formula I: ##STR3## in which R.sup.1 is alkyl, alkoxy, alkenyl or alkenyloxy having 1 to 12 carbon atoms, PA1 R.sup.b is preferably H or straight-chain alkyl having 1 to 3 carbon atoms. PA1 R.sup.* has the meaning given under the formula I.sup.* and is preferably a straight-chain alkyl group having 1 to 5 carbon atoms which is unsubstituted, monosubstituted by CF.sub.3 or at least monosubstituted by F. PA1 Q is --CF.sub.2 --, --CHF--, --OCF.sub.2 --, --OCHF-- or a single bond, PA1 Y is F or Cl, PA1 a is 1 or 2, and PA1 b is 0 or 1. PA1 Z.sup.4 is --CH.sub.2 CH.sub.2 --, --CO--O-- or a single bond, ##STR19## PA1 R.sup.5 is CH.sub.3 --(CH.sub.2).sub.p - or trans-H--(CH.sub.2).sub.q --CH.dbd.CH--(CH.sub.2 CH.sub.2).sub.b --, PA1 o is 1, 2, 3 or 4, PA1 q is 0, 1, 2 or 3, PA1 b is 0 or 1, and PA1 p is 1, 2, 3 or 4. PA1 R.sup.5 is CH.sub.3 --(CH.sub.2).sub.p --, where PA1 o is 1, 2, 3 or 4, PA1 q is 0, 1, 2 or 3, PA1 b is 0 or 1, and PA1 p is 1, 2, 3 or 4. PA1 u is from 1 to 12, PA1 Q is --CF.sub.2 --, --CHF--, --OCF.sub.2 --, --OCHF or a single bond, and PA1 Y is F or Cl, PA1 R.sup.4 and R.sup.5 are each, independently of one another, as defined for R. PA1 R.sup.5 is preferably an alkoxy radical having 1 to 7 carbon atoms, particularly preferably an alkoxy radical having 1 to 4 carbon atoms, especially an alkoxy radical having 1 to 2 carbon atoms. PA1 component A comprises at least two compounds selected from the compounds of the formula A1b1 and the formula A1c in which one of the radicals L.sup.3 and L.sup.4 is H and the other of these radicals is H or F; PA1 component B comprises one or more compounds selected from the formulae B4a and B12a: ##STR32## in which R.sup.4 and R.sup.5 have the preferred meanings given under the compounds of component B. The 1,4-phenylene groups in the compounds of the formula B12a can also be substituted by fluorine. The proportion of these compounds in the liquid-crystal mixtures is from 0 to 35%, preferably from 15 to 30%; PA1 component B comprises one or more compounds of the formula B3; PA1 the liquid-crystal mixture comprises one or more compounds in which R is trans-alkenyl or trans-alkenyloxy; PA1 component T comprises one or more compounds of the formulae T1 or T2. PA1 S-N smectic-nematic phase transition temperature PA1 N-I nematic-isotropic phase transition temperature PA1 Cl.p. clearing point PA1 visc. rotational viscosity (mPa.s) PA1 t.sub.on time from switching on until 90% of the maximum contrast is reached PA1 t.sub.off time from switching off until 10% of the maximum contrast is reached PA1 steepness ((V.sub.90 /V.sub.10)-1).100% PA1 p pitch PA1 V.sub.10 threshold voltage=characteristic voltage at a relative contrast of 10% (also abbreviated to V.sub.10, 0, 20)) PA1 V.sub.90 characteristic voltage at a relative contrast of 90%.
The parameter combinations achieved are still far from adequate, in particular for high-multiplex STNs (1/400). This is in some cases attributable to the fact that the various requirements are affected in opposite manners by material parameters.
There thus continues to be a great demand for SLCDs having very short response times and at the same time a large operating temperature range, high characteristic line steepness, good angle dependence of the contrast and low threshold voltage which meet the abovementioned requirements.
The invention has as an object providing SLCDs which do not have the above-mentioned disadvantages, or only do so to a lesser extent, and at the same time have very short response times.